Children as young as two years old will have to wear face coverings while visiting Disney theme parks.
All cast members and guests as young as two will need to put on a mask while indoors.
The guidance applies to locations at Walt Disney World Resort in Orlando, Florida, and Disneyland Resort in Anaheim, California.
The rules will come into effect from Friday.
In a statement park officials said: "We are adapting our health and safety guidelines based on guidance from health and government officials, and will require Cast Members and Guests ages two and up, to wear face coverings in all indoor locations at Walt Disney World Resort and Disneyland Resort beginning Friday, July 30, regardless of vaccination status.
"At Walt Disney World Resort, this includes upon entering and throughout all attractions and in enclosed transportation vehicles, including shuttles, buses, monorails and at Disney Skyliner."
The two Disney theme parks made the announcement on their official websites and said masks will remain optional for all guests in outdoor common areas.
Their previous policy was to allow fully vaccinated guests to stay exempt in most areas, while no official proof of having two doses of a vaccine was required by staff.
While face masks are no longer compulsory in many places in the UK, resorts operate their own Covid security policies.
In Chessington World of Adventures, the park has recommended face covering signage "in indoor or high footfall areas and some suggested social distancing markers".
Alton Towers say: "We will encourage face masks in certain areas – during your visit you will see some signage stating 'Face coverings recommended' which will be displayed at entry points into indoor or high footfall areas."
The change in park policy at Disney comes amid a surge of new Covid cases in many parts of the US.
The nation's Centers for Disease Control and Prevention blamed the more contagious Delta variant for the rise in cases.
It said infection numbers were going up in over 90 per cent of U.S. states & territories. The current 7-day average of daily new cases is 61,976.
This is a 64.1 per cent increase from the previous week.
In the last 24 hours alone there were 135,158 new cases of the virus recorded.
The CDC said that 66.6% of U.S. counties had transmission rates of Covid-19 high enough to warrant indoor masking.
US President Joe Biden is expected to soon announce that about two million civilian federal workers will need to be vaccinated or face testing, social distancing, mask requirements and limits on travel.
